Methods for driving electro-optic displays

ABSTRACT

There are provided display controllers and driving methods related to those described in US Published Patent Application No. 2013/0194250. These include (a) a display controller having an update buffer, means for removing from the update buffer pixels not requiring updating, and means to ensure that pixels having certain special states are not removed from the update buffer; (b) a method of driving a bistable display wherein, in a pixel undergoing a white-to-white transition and lying adjacent another pixel undergoing a visible transition, there is applied to the pixel one or more balanced pulse pairs and at least one top-off pulse; (c) a method of driving a bistable display by overlaying a non-rectangular item over a pre-existing image content and then removing the item, where only pixels in the region of the item perform transitions (including self-transitions); and (d) a method of driving a bistable display in which a proportion of background pixels not undergoing an optical change are subjected to a refresh pulse to correct optical state drift.

REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of non-provisionalapplication Ser. No. 14/447,707 filed on Jul. 31, 2014, which claimedthe benefit of provisional Application Ser. No. 61/861,137, filed Aug.1, 2013; of provisional Application Ser. No. 61,860,466, filed Jul. 31,2013; and of provisional Application Ser. No. 61/860,936 filed Aug. 1,2013.

This application is related to U.S. Pat. Nos. 5,930,026; 6,445,489;6,504,524; 6,512,354; 6,531,997; 6,753,999; 6,825,970; 6,900,851;6,995,550; 7,012,600; 7,023,420; 7,034,783; 7,116,466; 7,119,772;7,193,625; 7,202,847; 7,259,744; 7,304,787; 7,312,794; 7,327,511;7,453,445; 7,492,339; 7,528,822; 7,545,358; 7,583,251; 7,602,374;7,612,760; 7,679,599; 7,688,297; 7,729,039; 7,733,311; 7,733,335;7,787,169; 7,952,557; 7,956,841; 7,999,787; and 8,077,141; and U.S.Patent Applications Publication Nos. 2003/0102858; 2005/0122284;2005/0179642; 2005/0253777; 2006/0139308; 2007/0013683; 2007/0091418;2007/0103427; 2007/0200874; 2008/0024429; 2008/0024482; 2008/0048969;2008/0129667; 2008/0136774; 2008/0150888; 2008/0291129; 2009/0174651;2009/0179923; 2009/0195568; 2009/0256799; 2009/0322721; 2010/0045592;2010/0220121; 2010/0220122; 2010/0265561 2011/0285754, and 2013/0194250.This application is also related to copending Application Ser. No.14/445,641, filed Jul. 29, 2014 and claiming priority of provisionalApplication Ser. No.

The aforementioned patents and applications may hereinafter forconvenience collectively be referred to as the “MEDEOD” (MEthods forDriving Electro-Optic Displays) applications. The entire contents ofthese patents and copending applications, and of all other U.S. patentsand published and copending applications mentioned below, are hereinincorporated by reference.

BACKGROUND OF INVENTION

The present invention relates to methods for driving electro-opticdisplays, especially bistable electro-optic displays, and to apparatusfor use in such methods. More specifically, this invention relates todriving methods which may allow for reduced “ghosting” and edge effects,and reduced flashing in such displays. This invention is especially, butnot exclusively, intended for use with particle-based electrophoreticdisplays in which one or more types of electrically charged particlesare present in a fluid and are moved through the fluid under theinfluence of an electric field to change the appearance of the display.

The term “electro-optic”, as applied to a material or a display, is usedherein in its conventional meaning in the imaging art to refer to amaterial having first and second display states differing in at leastone optical property, the material being changed from its first to itssecond display state by application of an electric field to thematerial. Although the optical property is typically color perceptibleto the human eye, it may be another optical property, such as opticaltransmission, reflectance, luminescence or, in the case of displaysintended for machine reading, pseudo-color in the sense of a change inreflectance of electromagnetic wavelengths outside the visible range.

The term “gray state” is used herein in its conventional meaning in theimaging art to refer to a state intermediate two extreme optical statesof a pixel, and does not necessarily imply a black-white transitionbetween these two extreme states. For example, several of the E Inkpatents and published applications referred to below describeelectrophoretic displays in which the extreme states are white and deepblue, so that an intermediate “gray state” would actually be pale blue.Indeed, as already mentioned, the change in optical state may not be acolor change at all. The terms “black” and “white” may be usedhereinafter to refer to the two extreme optical states of a display, andshould be understood as normally including extreme optical states whichare not strictly black and white, for example the aforementioned whiteand dark blue states. The term “monochrome” may be used hereinafter todenote a drive scheme which only drives pixels to their two extremeoptical states with no intervening gray states.

The terms “bistable” and “bistability” are used herein in theirconventional meaning in the art to refer to displays comprising displayelements having first and second display states differing in at leastone optical property, and such that after any given element has beendriven, by means of an addressing pulse of finite duration, to assumeeither its first or second display state, after the addressing pulse hasterminated, that state will persist for at least several times, forexample at least four times, the minimum duration of the addressingpulse required to change the state of the display element. It is shownin U.S. Pat. No. 7,170,670 that some particle-based electrophoreticdisplays capable of gray scale are stable not only in their extremeblack and white states but also in their intermediate gray states, andthe same is true of some other types of electro-optic displays. Thistype of display is properly called “multi-stable” rather than bistable,although for convenience the term “bistable” may be used herein to coverboth bistable and multi-stable displays.

The term “impulse” is used herein in its conventional meaning of theintegral of voltage with respect to time. However, some bistableelectro-optic media act as charge transducers, and with such media analternative definition of impulse, namely the integral of current overtime (which is equal to the total charge applied) may be used. Theappropriate definition of impulse should be used, depending on whetherthe medium acts as a voltage-time impulse transducer or a charge impulsetransducer.

Much of the discussion below will focus on methods for driving one ormore pixels of an electro-optic display through a transition from aninitial gray level to a final gray level (which may or may not bedifferent from the initial gray level). The term “waveform” will be usedto denote the entire voltage against time curve used to effect thetransition from one specific initial gray level to a specific final graylevel. Typically such a waveform will comprise a plurality of waveformelements; where these elements are essentially rectangular (i.e., wherea given element comprises application of a constant voltage for a periodof time); the elements may be called “pulses” or “drive pulses”. Theterm “drive scheme” denotes a set of waveforms sufficient to effect allpossible transitions between gray levels for a specific display. Adisplay may make use of more than one drive scheme; for example, theaforementioned U.S. Pat. No. 7,012,600 teaches that a drive scheme mayneed to be modified depending upon parameters such as the temperature ofthe display or the time for which it has been in operation during itslifetime, and thus a display may be provided with a plurality ofdifferent drive schemes to be used at differing temperature etc. A setof drive schemes used in this manner may be referred to as “a set ofrelated drive schemes.” It is also possible, as described in several ofthe aforementioned MEDEOD applications, to use more than one drivescheme simultaneously in different areas of the same display, and a setof drive schemes used in this manner may be referred to as “a set ofsimultaneous drive schemes.”

Several types of electro-optic displays are known. One type ofelectro-optic display is a rotating bichromal member type as described,for example, in U.S. Pat. Nos. 5,808,783; 5,777,782; 5,760,761;6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791(although this type of display is often referred to as a “rotatingbichromal ball” display, the term “rotating bichromal member” ispreferred as more accurate since in some of the patents mentioned abovethe rotating members are not spherical). Such a display uses a largenumber of small bodies (typically spherical or cylindrical) which havetwo or more sections with differing optical characteristics, and aninternal dipole. These bodies are suspended within liquid-filledvacuoles within a matrix, the vacuoles being filled with liquid so thatthe bodies are free to rotate. The appearance of the display is changedby applying an electric field thereto, thus rotating the bodies tovarious positions and varying which of the sections of the bodies isseen through a viewing surface. This type of electro-optic medium istypically bistable.

Another type of electro-optic display uses an electrochromic medium, forexample an electrochromic medium in the form of a nanochromic filmcomprising an electrode formed at least in part from a semi-conductingmetal oxide and a plurality of dye molecules capable of reversible colorchange attached to the electrode; see, for example O'Regan, B., et al.,Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24(March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845.Nanochromic films of this type are also described, for example, in U.S.Pat. Nos. 6,301,038; 6,870,657; and 6,950,220. This type of medium isalso typically bistable.

Another type of electro-optic display is an electro-wetting displaydeveloped by Philips and described in Hayes, R. A., et al., “Video-SpeedElectronic Paper Based on Electrowetting”, Nature, 425, 383-385 (2003).It is shown in U.S. Pat. No. 7,420,549 that such electro-wettingdisplays can be made bistable.

One type of electro-optic display, which has been the subject of intenseresearch and development for a number of years, is the particle-basedelectrophoretic display, in which a plurality of charged particles movethrough a fluid under the influence of an electric field.Electrophoretic displays can have attributes of good brightness andcontrast, wide viewing angles, state bistability, and low powerconsumption when compared with liquid crystal displays. Nevertheless,problems with the long-term image quality of these displays haveprevented their widespread usage. For example, particles that make upelectrophoretic displays tend to settle, resulting in inadequateservice-life for these displays.

As noted above, electrophoretic media require the presence of a fluid.In most prior art electrophoretic media, this fluid is a liquid, butelectrophoretic media can be produced using gaseous fluids; see, forexample, Kitamura, T., et al., “Electrical toner movement for electronicpaper-like display”, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y.,et al., “Toner display using insulative particles chargedtriboelectrically”, IDW Japan, 2001, Paper AMD4-4). See also U.S. Pat.Nos. 7,321,459 and 7,236,291. Such gas-based electrophoretic mediaappear to be susceptible to the same types of problems due to particlesettling as liquid-based electrophoretic media, when the media are usedin an orientation which permits such settling, for example in a signwhere the medium is disposed in a vertical plane. Indeed, particlesettling appears to be a more serious problem in gas-basedelectrophoretic media than in liquid-based ones, since the lowerviscosity of gaseous suspending fluids as compared with liquid onesallows more rapid settling of the electrophoretic particles. [Para 17]Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT) and E Ink Corporationdescribe various technologies used in encapsulated electrophoretic andother electro-optic media. Such encapsulated media comprise numeroussmall capsules, each of which itself comprises an internal phasecontaining electrophoretically-mobile particles in a fluid medium, and acapsule wall surrounding the internal phase. Typically, the capsules arethemselves held within a polymeric binder to form a coherent layerpositioned between two electrodes. The technologies described in thethese patents and applications include:

-   -   (a) Electrophoretic particles, fluids and fluid additives; see        for example U.S. Pat. Nos. 7,002,728; and 7,679,814;    -   (b) Capsules, binders and encapsulation processes; see for        example U.S. Pat. Nos. 6,922,276; and 7,411,719;    -   (c) Films and sub-assemblies containing electro-optic materials;        see for example U.S. Pat. Nos. 6,982,178; and 7,839,564;    -   (d) Backplanes, adhesive layers and other auxiliary layers and        methods used in displays; see for example U.S. Pat. Nos.        7,116,318; and 7,535,624;    -   (e) Color formation and color adjustment; see for example U.S.        Pat. No. 7,075,502; and U.S. Patent Application Publication No.        2007/0109219;    -   (f) Methods for driving displays; see the aforementioned MEDEOD        applications;    -   (g) Applications of displays; see for example U.S. Pat. No.        7,312,784; and U.S. Patent Application Publication No.        2006/0279527; and    -   (h) Non-electrophoretic displays, as described in U.S. Pat. Nos.        6,241,921; 6,950,220; and 7,420,549; and U.S. Patent Application        Publication No. 2009/0046082.

Many of the aforementioned patents and applications recognize that thewalls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called polymer-dispersed electrophoretic display, inwhich the electrophoretic medium comprises a plurality of discretedroplets of an electrophoretic fluid and a continuous phase of apolymeric material, and that the discrete droplets of electrophoreticfluid within such a polymer-dispersed electrophoretic display may beregarded as capsules or microcapsules even though no discrete capsulemembrane is associated with each individual droplet; see for example,the aforementioned U.S. Pat. No. 6,866,760. Accordingly, for purposes ofthe present application, such polymer-dispersed electrophoretic mediaare regarded as sub-species of encapsulated electrophoretic media.

A related type of electrophoretic display is a so-called “microcellelectrophoretic display”. In a microcell electrophoretic display, thecharged particles and the fluid are not encapsulated withinmicrocapsules but instead are retained within a plurality of cavitiesformed within a carrier medium, typically a polymeric film. See, forexample, U.S. Pat. Nos. 6,672,921 and 6,788,449, both assigned to SipixImaging, Inc.

Although electrophoretic media are often opaque (since, for example, inmany electrophoretic media, the particles substantially blocktransmission of visible light through the display) and operate in areflective mode, many electrophoretic displays can be made to operate ina so-called “shutter mode” in which one display state is substantiallyopaque and one is light-transmissive. See, for example, U.S. Pat. Nos.5,872,552; 6,130,774; 6,144,361; 6,172,798; 6,271,823; 6,225,971; and6,184,856. Dielectrophoretic displays, which are similar toelectrophoretic displays but rely upon variations in electric fieldstrength, can operate in a similar mode; see U.S. Pat. No. 4,418,346.Other types of electro-optic displays may also be capable of operatingin shutter mode. Electro-optic media operating in shutter mode may beuseful in multi-layer structures for full color displays; in suchstructures, at least one layer adjacent the viewing surface of thedisplay operates in shutter mode to expose or conceal a second layermore distant from the viewing surface.

An encapsulated electrophoretic display typically does not suffer fromthe clustering and settling failure mode of traditional electrophoreticdevices and provides further advantages, such as the ability to print orcoat the display on a wide variety of flexible and rigid substrates.(Use of the word “printing” is intended to include all forms of printingand coating, including, but without limitation: pre-metered coatingssuch as patch die coating, slot or extrusion coating, slide or cascadecoating, curtain coating; roll coating such as knife over roll coating,forward and reverse roll coating; gravure coating; dip coating; spraycoating; meniscus coating; spin coating; brush coating; air knifecoating; silk screen printing processes; electrostatic printingprocesses; thermal printing processes; ink jet printing processes;electrophoretic deposition (See U.S. Pat. No. 7,339,715); and othersimilar techniques.) Thus, the resulting display can be flexible.Further, because the display medium can be printed (using a variety ofmethods), the display itself can be made inexpensively.

Other types of electro-optic media may also be used in the displays ofthe present invention.

The bistable or multi-stable behavior of particle-based electrophoreticdisplays, and other electro-optic displays displaying similar behavior(such displays may hereinafter for convenience be referred to as“impulse driven displays”), is in marked contrast to that ofconventional liquid crystal (“LC”) displays. Twisted nematic liquidcrystals are not bi- or multi-stable but act as voltage transducers, sothat applying a given electric field to a pixel of such a displayproduces a specific gray level at the pixel, regardless of the graylevel previously present at the pixel. Furthermore, LC displays are onlydriven in one direction (from non-transmissive or “dark” to transmissiveor “light”), the reverse transition from a lighter state to a darker onebeing effected by reducing or eliminating the electric field. Finally,the gray level of a pixel of an LC display is not sensitive to thepolarity of the electric field, only to its magnitude, and indeed fortechnical reasons commercial LC displays usually reverse the polarity ofthe driving field at frequent intervals. In contrast, bistableelectro-optic displays act, to a first approximation, as impulsetransducers, so that the final state of a pixel depends not only uponthe electric field applied and the time for which this field is applied,but also upon the state of the pixel prior to the application of theelectric field.

Whether or not the electro-optic medium used is bistable, to obtain ahigh-resolution display, individual pixels of a display must beaddressable without interference from adjacent pixels. One way toachieve this objective is to provide an array of non-linear elements,such as transistors or diodes, with at least one non-linear elementassociated with each pixel, to produce an “active matrix” display. Anaddressing or pixel electrode, which addresses one pixel, is connectedto an appropriate voltage source through the associated non-linearelement. Typically, when the non-linear element is a transistor, thepixel electrode is connected to the drain of the transistor, and thisarrangement will be assumed in the following description, although it isessentially arbitrary and the pixel electrode could be connected to thesource of the transistor. Conventionally, in high resolution arrays, thepixels are arranged in a two-dimensional array of rows and columns, suchthat any specific pixel is uniquely defined by the intersection of onespecified row and one specified column. The sources of all thetransistors in each column are connected to a single column electrode,while the gates of all the transistors in each row are connected to asingle row electrode; again the assignment of sources to rows and gatesto columns is conventional but essentially arbitrary, and could bereversed if desired. The row electrodes are connected to a row driver,which essentially ensures that at any given moment only one row isselected, i.e., that there is applied to the selected row electrode avoltage such as to ensure that all the transistors in the selected roware conductive, while there is applied to all other rows a voltage suchas to ensure that all the transistors in these non-selected rows remainnon-conductive. The column electrodes are connected to column drivers,which place upon the various column electrodes voltages selected todrive the pixels in the selected row to their desired optical states.(The aforementioned voltages are relative to a common front electrodewhich is conventionally provided on the opposed side of theelectro-optic medium from the non-linear array and extends across thewhole display.) After a pre-selected interval known as the “line addresstime” the selected row is deselected, the next row is selected, and thevoltages on the column drivers are changed so that the next line of thedisplay is written. This process is repeated so that the entire displayis written in a row-by-row manner.

It might at first appear that the ideal method for addressing such animpulse-driven electro-optic display would be so-called “generalgrayscale image flow” in which a controller arranges each writing of animage so that each pixel transitions directly from its initial graylevel to its final gray level. However, inevitably there is some errorin writing images on an impulse-driven display. Some such errorsencountered in practice include:

-   -   (a) Prior State Dependence; With at least some electro-optic        media, the impulse required to switch a pixel to a new optical        state depends not only on the current and desired optical state,        but also on the previous optical states of the pixel.    -   (b) Dwell Time Dependence; With at least some electro-optic        media, the impulse required to switch a pixel to a new optical        state depends on the time that the pixel has spent in its        various optical states. The precise nature of this dependence is        not well understood, but in general, more impulse is required        the longer the pixel has been in its current optical state.    -   (c) Temperature Dependence; The impulse required to switch a        pixel to a new optical state depends heavily on temperature.    -   (d) Humidity Dependence; The impulse required to switch a pixel        to a new optical state depends, with at least some types of        electro-optic media, on the ambient humidity.    -   (e) Mechanical Uniformity; The impulse required to switch a        pixel to a new optical state may be affected by mechanical        variations in the display, for example variations in the        thickness of an electro-optic medium or an associated lamination        adhesive. Other types of mechanical non-uniformity may arise        from inevitable variations between different manufacturing        batches of medium, manufacturing tolerances and materials        variations.    -   (f) Voltage Errors; The actual impulse applied to a pixel will        inevitably differ slightly from that theoretically applied        because of unavoidable slight errors in the voltages delivered        by drivers.

General grayscale image flow suffers from an “accumulation of errors”phenomenon. For example, imagine that temperature dependence results ina 0.2 L* (where L* has the usual CIE definition:

L*=116(R/R ₀)^(1/3)−16,

where R is the reflectance and R₀ is a standard reflectance value) errorin the positive direction on each transition. After fifty transitions,this error will accumulate to 10 L*. Perhaps more realistically, supposethat the average error on each transition, expressed in terms of thedifference between the theoretical and the actual reflectance of thedisplay is ±0.2 L*. After 100 successive transitions, the pixels willdisplay an average deviation from their expected state of 2 L*; suchdeviations are apparent to the average observer on certain types ofimages.

This accumulation of errors phenomenon applies not only to errors due totemperature, but also to errors of all the types listed above. Asdescribed in the aforementioned U.S. Pat. No. 7,012,600, compensatingfor such errors is possible, but only to a limited degree of precision.For example, temperature errors can be compensated by using atemperature sensor and a lookup table, but the temperature sensor has alimited resolution and may read a temperature slightly different fromthat of the electro-optic medium. Similarly, prior state dependence canbe compensated by storing the prior states and using a multi-dimensionaltransition matrix, but controller memory limits the number of statesthat can be recorded and the size of the transition matrix that can bestored, placing a limit on the precision of this type of compensation.

Thus, general grayscale image flow requires very precise control ofapplied impulse to give good results, and empirically it has been foundthat, in the present state of the technology of electro-optic displays,general grayscale image flow is infeasible in a commercial display.

Under some circumstances, it may be desirable for a single display tomake use of multiple drive schemes. For example, a display capable ofmore than two gray levels may make use of a gray scale drive scheme(“GSDS”) which can effect transitions between all possible gray levels,and a monochrome drive scheme (“MDS”) which effects transitions onlybetween two gray levels, the MDS providing quicker rewriting of thedisplay that the GSDS. The MDS is used when all the pixels which arebeing changed during a rewriting of the display are effectingtransitions only between the two gray levels used by the MDS. Forexample, the aforementioned U.S. Pat. No. 7,119,772 describes a displayin the form of an electronic book or similar device capable ofdisplaying gray scale images and also capable of displaying a monochromedialogue box which permits a user to enter text relating to thedisplayed images. When the user is entering text, a rapid MDS is usedfor quick updating of the dialogue box, thus providing the user withrapid confirmation of the text being entered. On the other hand, whenthe entire gray scale image shown on the display is being changed, aslower GSDS is used.

Alternatively, a display may make use of a GSDS simultaneously with a“direct update” drive scheme (“DUDS”). The DUDS may have two or morethan two gray levels, typically fewer than the GSDS, but the mostimportant characteristic of a DUDS is that transitions are handled by asimple unidirectional drive from the initial gray level to the finalgray level, as opposed to the “indirect” transitions often used in aGSDS, where in at least some transitions the pixel is driven from aninitial gray level to one extreme optical state, then in the reversedirection to a final gray level (this type of waveform may forconvenience be referred to as a “single rail bounce” waveform); in somecases, the transition may be effected by driving from the initial graylevel to one extreme optical state, thence to the opposed extremeoptical state, and only then to the final extreme optical state (thistype of waveform may for convenience be referred to as a “double railbounce” waveform)—see, for example, the drive scheme illustrated inFIGS. 11A and 11B of the aforementioned U.S. Pat. No. 7,012,600. Presentelectrophoretic displays may have an update time in grayscale mode ofabout two to three times the length of a saturation pulse (where “thelength of a saturation pulse” is defined as the time period, at aspecific voltage, that suffices to drive a pixel of a display from oneextreme optical state to the other), or approximately 700-900milliseconds, whereas a DUDS has a maximum update time equal to thelength of the saturation pulse, or about 200-300 milliseconds.

Variation in drive schemes is, however, not confined to differences inthe number of gray levels used. For example, drive schemes may bedivided into global drive schemes, where a drive voltage is applied toevery pixel in the region to which the global update drive scheme (moreaccurately referred to as a “global complete” or “GC” drive scheme) isbeing applied (which may be the whole display or some defined portionthereof) and partial update drive schemes, where a drive voltage isapplied only to pixels that are undergoing a non-zero transition (i.e.,a transition in which the initial and final gray levels differ from eachother), but no drive voltage is applied during zero transitions (inwhich the initial and final gray levels are the same). An intermediateform a drive scheme (designated a “global limited” or “GL” drive scheme)is similar to a GC drive scheme except that no drive voltage is appliedto a pixel which is undergoing a zero, white-to-white transition. In,for example, a display used as an electronic book reader, displayingblack text on a white background, there are numerous white pixels,especially in the margins and between lines of text which remainunchanged from one page of text to the next; hence, not rewriting thesewhite pixels substantially reduces the apparent “flashiness” of thedisplay rewriting. However, certain problems remain in this type of GLdrive scheme. Firstly, as discussed in detail in some of theaforementioned MEDEOD applications, bistable electro-optic media aretypically not completely bistable, and pixels placed in one extremeoptical state gradually drift, over a period of minutes to hours,towards an intermediate gray level. In particular, pixels driven whiteslowly drift towards a light gray color. Hence, if in a GL drive schemea white pixel is allowed to remain undriven through a number of pageturns, during which other white pixels (for example, those forming partsof the text characters) are driven, the freshly updated white pixelswill be slightly lighter than the undriven white pixels, and eventuallythe difference will become apparent even to an untrained user.

Secondly, when an undriven pixel lies adjacent a pixel which is beingupdated, a phenomenon known as “blooming” occurs, in which the drivingof the driven pixel causes a change in optical state over an areaslightly larger than that of the driven pixel, and this area intrudesinto the area of adjacent pixels. Such blooming manifests itself as edgeeffects along the edges where the undriven pixels lie adjacent drivenpixels. Similar edge effects occur when using regional updates (whereonly a particular region of the display is updated, for example to showan image), except that with regional updates the edge effects occur atthe boundary of the region being updated. Over time, such edge effectsbecome visually distracting and must be cleared. Edge ghosting isclearly visible for example after multiple text page updates followed byan update to a white page where the contour of the previous text willappear darker compared to the background. Hitherto, such edge effects(and the effects of color drift in undriven white pixels) have typicallybeen removed by using a single GC update at intervals. Unfortunately,use of such an occasional GC update reintroduces the problem of a“flashy” update, and indeed the flashiness of the update may beheightened by the fact that the flashy update only occurs at longintervals.

The aforementioned US 2013/0194250 describes techniques for reducingflashing and edge ghosting. One such technique, denoted a “selectivegeneral update” or “SGU” method, involves driving an electro-opticdisplay having a plurality of pixels using a first drive scheme, inwhich all pixels are driven at each transition, and a second drivescheme, in which pixels undergoing some transitions are not driven. Thefirst drive scheme is applied to a non-zero minor proportion of thepixels during a first update of the display, while the second drivescheme is applied to the remaining pixels during the first update.During a second update following the first update, the first drivescheme is applied to a different non-zero minor proportion of thepixels, while the second drive scheme is applied to the remaining pixelsduring the second update. Typically, the SGU method is applied torefreshing the white background surrounding text or an image, so thatonly a minor proportion of the pixels in the white background undergoupdating during any one display update, but all pixels of the backgroundare gradually updated so that drifting of the white background to a graycolor is avoided without any need for a flashy update. It will readilybe apparent to those skilled in the technology of electro-optic displaysthat application of the SGU method requires a special waveform(hereinafter referred to as an “F” waveform) for the individual pixelswhich are to undergo updating on each transition.

The aforementioned US 2013/0194250 also describes a “balanced pulse pairwhite/white transition drive scheme” or “BPPWWTDS”, which involves theapplication of one or more balanced pulse pairs (a balanced pulse pairor “BPP” being a pair of drive pulses of opposing polarities such thatthe net impulse of the balanced pulse pair is substantially zero) duringwhite-to-white transitions in pixels which can be identified as likelyto give rise to edge artifacts, and are in a spatio-temporalconfiguration such that the balanced pulse pair(s) will be efficaciousin erasing or reducing the edge artifact. Desirably, the pixels to whichthe BPP is applied are selected such that the BPP is masked by otherupdate activity. Note that application of one or more BPP's does notaffect the desirable DC balance of a drive scheme since each BPPinherently has zero net impulse and thus does not alter the DC balanceof a drive scheme. A second such technique, denoted “white/white top-offpulse drive scheme” or “WWTOPDS”, involves applying a “top-off” pulseduring white-to-white transitions in pixels which can be identified aslikely to give rise to edge artifacts, and are in a spatio-temporalconfiguration such that the top-off pulse will be efficacious in erasingor reducing the edge artifact. Application of the BPPWWTDS or WWTOPDSagain requires a special waveform (hereinafter referred to as a “T”waveform) for the individual pixels which are to undergo updating oneach transition. The T and F waveforms are normally only applied topixels undergoing white-to-white transitions. In a global limited drivescheme, the white-to-white waveform is empty (i.e., consists of a seriesof zero voltage pulses) whereas all other waveforms are not empty.Accordingly, when applicable the non-empty T and F waveforms replace theempty white-to-white waveforms in a global limited drive scheme.

It is necessary to distinguish between a partial update drive scheme (inwhich at least some zero transitions use empty waveforms) and a partialupdate mode of a drive controller. A partial update mode is a controllerfunction by which, when active, a pixel is removed from the updatepipeline if it is a zero transition. For example if the initial state ofa pixel was gray level 7 and the final state is also gray level 7, thenthat pixel will not be assigned to a transition pipeline and will befree to participate in another subsequent update at any time. In apartial update mode, only areas in the next image buffer that differfrom the current image buffer will be driven by the drive scheme. Thisis especially helpful when overlaying an item (such as an icon, cursoror menu over an existing image (typically text); the overlaid can bestamped into the image buffer and sent to the controller, but only thearea of the overlaid item will flash.

As already indicated, partial update behavior can also be expressed bydrive scheme design. For example, a global limited (GL) mode may have anempty white-to-white transition but non-empty gray-to-gray andblack-to-black transitions, so a white background will not flash whenoverlaying a menu, but the non-white text would flash. Other waveformmodes such as DU and GU have all empty zero-transitions. In this casethe behavior of the display will be exactly as described for partialupdate mode, but with one important difference: the zero transitionpixels are not removed from the pipeline and must be driven with zerosfor the full duration of the update.

One can envision a selective partial update mode in which a zerotransition pixel may or may not be removed from the update pipeline (oralternatively, receive a zero transition waveform) depending on analgorithmic decision. This concept may be generalized in the followingmanner. Each pixel of the display has an associated flag indicatingwhether that pixel does or does not receive an appropriate waveform. Theflags define a Partial Update Mask (PUM) for the whole image in whichflags are set TRUE for pixels that are driven and FALSE for pixels whichare not driven. Any pixel undergoing a non-zero transition has a TRUEflag, but pixels undergoing zero transitions may have TRUE or FALSEflags.

Some issues arise the aforementioned when the T- and F-transitions areused with partial updates. Firstly, two additional device controllerstates are required to enable the T- and F-transitions. For simplicityassume states 1-16 correspond to the normal 16 gray levels, while state17 denotes a T transition and state 18 an F transition. A drive schemeis defined to convert any one initial state into any one final state. Inone form of the method, the final image buffer is preprocessed todetermine when to substitute state 17 or 18 for state 16 (correspondingto a white gray level) according to the algorithm being used. Thepreprocessed image is then sent to the display controller, where partialupdate logic is applied to remove pixels undergoing zero transitionsfrom the update pipeline. Pixels undergoing 16->16 (normalwhite-to-white) transitions can be removed from the pipeline since thattransition is empty in a GL mode. However, pixels undergoing 17->17 or18->18 transitions should not be removed from the pipeline since it ispossible that the algorithm would need to apply two T or two Ftransitions successively to the same pixel. One aspect of the presentinvention provides a means of achieving this aim in both the controllerand waveform implementations.

A more difficult issue is that the decisions to use T or F transitionson a pixel is based on the initial and final states of pixels adjacentto the pixel being considered; in particular, in some cases ifneighboring pixels are undergoing non-zero transitions, the decision asto whether to use T or F transitions on the pixel being considered canbe changed. The use of a partial update mode can thwart the ability ofthe algorithm to correctly identify neighboring pixels not undergoingnon-zero transitions, which can lead to reduced efficacy or even theintroduction of new artifacts.

A controller may also make use of a “regional update mode”; this mode issimilar to a partial update mode except that only pixels within aselected region of the display are placed on the update pipeline. Aregional update mode can be considered to be a specific case of aselective partial update mode in which the Partial Update Mask is set toFALSE for any pixel outside of the selected region. However, regionalupdates require special handling, as described below, because typicallyonly data for the selected region is transferred to the controller.

A second aspect of the present invention relates to improvingperformance of displays at temperatures above room temperature, forexample when a display in the form of an electronic book reader is beingused outdoors in summer. As already mentioned, US 2013/0194250 describesa “balanced pulse pair white/white transition drive scheme” or“BPPWWTDS”. In some cases, for example some electrophoretic displaysoperated at temperatures of 30° C. and above, a BPPWWTDS has been shownto be ineffective in reducing all edge artifacts. FIG. 1 shows thatusing a BPPWWTDS at 31° C. and 35° C. results in near zero improvementin edge ghosting scores after ten iterations with the use of a BPPWWTDSas compared with use of a drive scheme lacking balanced pulse pairs.While it has been observed that a BPPWWTDS is effective in reducing edgeghosting in the area between adjacent pixels as seen in FIG. 2 (whichshows a photomicrograph illustrating edge artifacts observed at 45° C.after ten iterations of driving a block of pixels to black, asillustrated on the left side of FIG. 2, followed by driving the sameblock of pixels to white, as illustrated on the right side, with use ofa BPPWWTDS on the neighboring pixels), the issue is that additionaleffects occurring at these temperatures result in edge artifacts that aBPPWWTDS is ineffective in reducing. For example, when a pixel isupdated from white to black with inactive neighbors, one-pixel-wide edgeartifacts are observed in its neighbors in the form of pixel darkeningand outer edge formation which can be described as high irreversibleblooming and can be explained by lateral coupling. These effects buildup with the number of updates and quickly result in significantdarkening in neighboring pixels. In display operation modes using aBPPWWTDS, such effects can result in significant decrease inperformance. For example, in a low flash mode using a BPPWWTDS aimed atmaintaining the background white state lightness level, such effectsresult in unacceptably high decrease in white state lightness level ofover 3L* after 24 updates at 45° C. as seen in FIG. 3.

The second aspect of the present invention relates to a DC imbalanceddrive scheme intended to significantly reduce the aforementioneddisadvantages of the BPPWWTDS.

A third aspect of the invention relates to improved selective partialupdate drive schemes. As mentioned above, electro-optic displays can bedriven using partial updates, in which all pixels with any“self-transition” (zero transition) going from one image to the next(meaning that the pixel goes from a specific gray level in one image tothe same specific gray level in the subsequent image), are not driven or(which amounts to the same thing) are driven with a waveform having avoltage list of zeros. Partial updates may be performed using a specialwaveform in which all the self-transitions are empty, i.e. filled withzeros, (usually called a “local” waveform) or using a device commandthat automatically detects self-transitions (known as a “partial updatemode”).

Partial updates offer benefits in terms of reduced display flashiness.For example, with an initial image that has some text, a partial updatemay be used if we want to overlay a menu option on top of the text inorder to avoid seeing the text updating on to itself. However, partialupdates can create problems and/or be undesirable and incompatible withcertain drive schemes. For example, consider a menu overlying existingtext that is displayed and then dismissed as illustrated in FIGS. 9A-9C.If a partial update drive scheme is used, in text that overlaps with theborder of the menu (as shown in FIG. 9B), all pixels withself-transition will be driven with an empty waveform, while theirneighbors may be performing transitions with a non-zero waveform, e.g. avoltage list effecting a white-to-black switch from the first image tothe second, followed by a black-to-white switch from the second image(FIG. 9B) to the third (FIG. 9C). These neighboring pixels may bloomover the undriven self-transition pixels, resulting in visually apparenttext thinning or text fading, as illustrated in the third image.

As described in some of the aforementioned MEDEOD applications,electro-optic displays may also be driven using regional updates, inwhich only pixels within a selected region of the display (this regionmay be rectangular or of an arbitrary shape, including being selectedpixel by pixel) are driven.

The third aspect of the present invention relates to drivingelectro-optic displays using selective partial update drive schemeswhich permit retention of the benefits of partial update drive schemesin terms of reduced flashiness without creating text thinning/fading andwith full compatibility with novel display modes for optimal displayperformance.

A fourth aspect of the present invention relates to drift compensation,that is to say compensating for changes in the optical state of anelectro-optic display with time. As already noted, electrophoretic andsimilar electro-optic displays are bistable. However, the bistability ofsuch displays is not unlimited in practice, a phenomenon known as imagedrift occurs, whereby pixels in or near extreme optical states tend torevert very slowly to intermediate gray levels; for example, blackpixels gradually become dark gray and white pixels gradually becomelight gray. The white state drift is of particular interest because manyelectro-optic displays are most commonly used to display images with awhite background; for example, electronic book readers normally mimicprinted books by displaying black text on a white background. If anelectro-optic display is updated using a global limited drive scheme fora long periods of time without a full display refresh, the white statedrift becomes an essential part of the overall visual appearance of thedisplay. Over time, the display will show areas of the display where thewhite state has been recently rewritten and other areas such as thebackground where the white state has not recently been rewritten and hasthus been drifting for some time. This results an optical artifact knownas ghosting, whereby the display shows traces of previous images. Suchghosting effects are sufficiently annoying to most users that theirpresence a significant part in preventing the use of global limiteddrive schemes exclusively for long periods of time.

FIG. 13 shows an example of how the background white state of a displaymay drift over the course of about twenty minutes, resulting insignificant ghosting as illustrated in FIG. 14, which shows an imageafter turning 45 text pages in low flash mode with 30 seconds betweenpage turns. In the last image as illustrated in FIG. 14, the text pagehas just been updated to a white page, and shows the ghosting resultingfrom “new” white in the text area versus “old” white in the background.

The fourth aspect of the present invention relates to a method fordriving a display which reduces or eliminates the problems caused bydrift without producing the flash which would be perceived if allbackground pixels were updated simultaneously as in a global completedrive scheme.

SUMMARY OF INVENTION

The first aspect of the present invention (which may hereinafter bereferred to as the “Update Buffer Invention”) provides a displaycontroller (capable of controlling the operation of a bistableelectro-optic display) having an update buffer, means for removing fromthe update buffer pixels which do not require updating during a giventransition, means for receiving a list of states that should not beremoved from the update buffer, and means to ensure that pixels havinglisted states are not removed from the update buffer.

The first aspect of the present invention also provides a displaycontroller capable of controlling the operation of a bistableelectro-optic display and having an update buffer, and means forremoving from the update buffer pixels which do not require updatingduring a given transition, the controller having at least one specialtransition having two states associated therewith, means to determinewhen a pixel is undergoing a special transition immediately after aprevious special transition, and means to insert into the update bufferthe second state associated with the at least one special transitionwhen a pixel is undergoing a special transition immediately after aprevious special transition.

The first aspect of the present invention also provides a drive schemewhich achieves essentially the same result as the display controllers ofthe present invention already mentioned. In such a drive scheme, zerotransitions use empty waveform, but zero transitions corresponding tothe special states use non-empty waveforms. This approach can work wellfor limited cases such as turning text pages, or going through an imagesequence in which each successive image is always different from theprevious one, or displaying and dismissing single items (icons, menus,etc.) that do not overlap with any of the non-white content of theinitial image, or browsing up and down through an existing menu.

The first aspect of the present invention also provides a modifiedalgorithm for carrying out the SGU, BPPWWTDS or WWTOPDS drive schemesdiscussed above to take into account the non-flashing pixels that willbe introduced by the partial update mode of the controller. First, thePartial Update Mask (PUM) value for each pixel must be computedaccording to the known controller algorithm. In the simplest case(standard partial update) the PUM is set to False if and only if theinitial and final gray levels in the image buffer are the same. Second,a modified algorithm is used which utilizes the PUM to determine localactivity as prescribed by the algorithm.

A second aspect of the present invention (which may hereinafter bereferred to as the “BPPTOPWWTDS Invention”) in effect combines theaforementioned BPPWWTDS and WWTOPDS by applying to pixels undergoingwhite-to-white transitions, identified as likely to give rise to edgeartifacts, and in a spatio-temporal configuration such that the drivescheme of the present invention will be efficacious in erasing orreducing the edge artifact, a waveform which comprises at least onebalanced pulse pair and at least one top-off pulse. This drive scheme ofthe present invention may for convenience be referred to a “balancedpulse pair/top-off pulse white/white transition drive scheme” or“BPPTOPWWTDS”.

The BPPTOPWWTDS of the present invention may be applied only when adisplay is operating in a particular temperature range, for example 30°C. or higher, where a prior art BPPWWTDS is ineffective. The BPPTOPWWTDSwaveform for white-to-white transitions may comprise a variable numberof balanced pulse pairs at varying locations within the waveform and avariable number of top-off pulses which may vary in location within thewaveform relative to the balanced pulse pairs. A single top-off pulsetypically corresponds to one frame of white-going drive pulse. Thetop-off pulse(s) may be located before, after or between the balancedpulse pairs. It is generally preferred that there be only a singletop-of pulse in the white-to-white transition waveform.

A third aspect of the present invention (which may hereinafter bereferred to as the “Overlay Invention”) is intended to be applied whenoverlaying an item (an icon, menu, etc.) (typically a non-rectangularitem) over existing text or image content followed by a removal of theitem as generally illustrated in FIGS. 10A-10C. The overlay method ofthe present invention differs from standard partial updates driveschemes in that only the pixels in the region of the item performtransitions (including self-transitions) in order to avoid textthinning/fading for text that overlaps with the item and to avoid seeingthe text outside that area flashing on to itself. One solution is toperform a regional update in the area of the item. Knowing the itemgeometry and location on the image, the controller can be used toperform a regional update for this area only.

This simple overlay method of the present invention is not well adaptedto cover situations in which the overlaid item is not completely opaque,i.e. the item does not fully fill a rectangle, or other area lyingwithin the boundary of the overlaid item. If there are areas intended tobe transparent within the overlaid item, they will also be fully updatedby the regional drive scheme, which is not desirable for the reasonsdiscussed above. An example of such a scenario is illustrated in FIGS.11A-11C.

To cope with this type of overlaid item, a preferred method of thepresent invention updates only the pixels that overlap with thenon-transparent (black as illustrated in FIGS. 11A-11C) portions of theoverlaid item (including such pixels undergoing self transitions), toproduce the second and third images, thus reducing or eliminatingvisible text thinning/fading. All the pixels with self transitions thatdo not overlap with the non-transparent (black) portion of the overlaiditem are updated with empty self transitions in order to reduceflashiness and avoid most of the text updating on to itself. This meansthat some black→black transitions are empty (for pixels not overlappingwith the non-transparent portions of the overlaid item) and some arenon-empty (for all other pixels) when updating to the second (FIG. 11B)and third (FIG. 11C) images.

In some methods of the present invention, it may be advantageous to takeadvantage of a drive scheme having waveforms for a number of gray levelsgreater than are actually present on the display. For example, if thedisplay uses only different gray levels, the drive scheme may be afive-bit (32 gray level) drive scheme which takes advantage of the extra“empty” space inside the drive scheme to cope with the differingblack-to-black transitions discussed above. A five-bit drive schemeallows 32 states, so that each of the 16 gray levels can use twodifferent states. For example, assuming states 1→32, gray level blackcan use state 1 which is set with a non-empty self transition (1→1) aswell as state 2 which is set with an empty self transition (2→2).Transition 1→2 is empty and transition 2→1 is a full non-emptyblack→black transition. From the overlaid item, the drive schemealgorithm determines the mask of pixels that must perform non-emptyself-transitions as illustrated in FIGS. 12A-12C. In FIG. 12A blackdenotes the pixels performing self transitions from FIG. 11A to FIG.11B, while in FIG. 12B black denotes all pixels that are underneath theoverlaid item, i.e., all pixels that in non-transparent portions of theoverlaid item. ANDing (in the Boolean sense of that term) the masks ofFIGS. 12A and 12B produces the mask of FIG. 12C, in which black denotesall pixels performing self transitions that need to be updated with anon-empty waveform. Using the mask of FIG. 12C, gray level black pixelsin the second image are processed so that all the black pixels in FIG.12C stay in state 1, while all other black pixels become state 2. Thismask-based algorithm allows all 16 gray levels to perform empty selftransitions in some areas and non-empty self transitions in other areas,thus in effect moving back and forth at the pixel level between apartial update mode and a full update mode.

As discussed above and in the aforementioned MEDEOD applications, aparticular drive scheme may be used in only certain regions of thedisplay, which may be rectangular or of arbitrary shape. The presentinvention thus extends to a driving method and controller in which aBPPTOPWWTDS is used in only one of a plurality of regions of a display.

A fourth aspect of the invention (which may hereinafter be referred toas the “Drift Compensation Invention”) provides a method of driving abistable electro-optic display having a plurality of pixels each capableof displaying two extreme optical states, the method comprising:

-   -   writing a first image on the display;    -   writing a second image on the display using a drive scheme in        which a plurality of background pixels which are in the same        extreme optical state in both the first and second images are        not driven;    -   leaving the display undriven for a period of time, thereby        permitting the background pixels to assume an optical state        different from their extreme optical state;    -   after said period of time, applying to a first non-zero        proportion of the background pixels a refresh pulse which        substantially restores the pixels to which it is applied to        their extreme optical state, said refresh pulse not being        applied to the background pixels other than said first non-zero        proportion thereof; and    -   thereafter, applying to a second non-zero minor proportion of        the background pixels different from the first non-zero        proportion a refresh pulse which substantially restores the        pixels to which it is applied to their extreme optical state,        said refresh pulse not being applied to the background pixels        other than said second non-zero proportion thereof.

In a preferred form of this drift compensation method, the display isprovided with a timer which establishes a minimum time interval (forexample, at least about 10 seconds, and typically at least about 60seconds) between successive applications of the refresh pulses todiffering non-zero proportions of the background pixels. As alreadyindicated, the drift compensation method will typically be applied tobackground pixels in the white extreme optical state but we do notexclude its application to background pixels in the black extremeoptical state, or in both extreme optical states. The drift compensationmethod may of course be applied to both monochrome and gray scaledisplays.

The present invention also provides novel display controllers arrangedto carry out all the methods of the present invention.

In the methods of the present invention, the display may make use of anyof the type of electro-optic media discussed above. Thus, for example,the electro-optic display may comprise a rotating bichromal member,electrochromic or electro-wetting material. Alternatively, theelectro-optic display may comprise an electrophoretic materialcomprising a plurality of electrically charged particles disposed in afluid and capable of moving through the fluid under the influence of anelectric field. The electrically charged particles and the fluid may beconfined within a plurality of capsules or microcells. Alternatively,the electrically charged particles and the fluid may be present as aplurality of discrete droplets surrounded by a continuous phasecomprising a polymeric material. The fluid may be liquid or gaseous.

The displays of the present invention may be used in any application inwhich prior art electro-optic displays have been used. Thus, forexample, the present displays may be used in electronic book readers,portable computers, tablet computers, cellular telephones, smart cards,signs, watches, shelf labels, variable transmission windows and flashdrives.

BRIEF DESCRIPTION OF THE DRAWINGS

As already mentioned, FIG. 1 of the accompanying drawings illustratesthe effectiveness of a prior art BPPWWTDS at various temperatures.

FIG. 2 is a photomicrograph illustrating edge artifacts observed afterdriving a block of pixels to black, followed by driving the same blockof pixels to white, with use of a BPPWWTDS on the neighboring pixels.

FIG. 3 illustrates white state lightness as function of the number ofupdates using a prior art BPPWWTDS.

FIG. 4 is a voltage against time curve for a BPPTOPWWTDS waveform forwhite-to-white transitions.

FIG. 5 is a photomicrograph similar to that of FIG. 2 but using aBPPTOPWWTDS of the present invention.

FIG. 6 is a graph similar to that of FIG. 3 but showing the resultsobtained using both a prior art BPPWWTDS and a BPPTOPWWTDS of thepresent invention.

FIG. 7 is a graph showing the white state lightness variation obtainedafter 24 updates from an initial white state lightness level as afunction of the number of updates using various BPPTOPWWTDS of thepresent invention.

FIG. 8 is a graph showing the gray levels obtained after more than50,000 updates using a BPPTOPWWTDS of the present invention.

As already mentioned, FIG. 9A shows a portion of a text image on adisplay.

FIG. 9B illustrates the effect of overlaying a menu over the text imageof FIG. 9A.

FIG. 9C illustrates the image resulting from subsequent removal of themenu shown in FIG. 9B.

FIG. 10A shows a text image on a display.

FIG. 10B illustrates the effect of overlaying an icon over the textimage of FIG. 10A.

FIG. 10C illustrates the image resulting from subsequent removal of theicon shown in FIG. 10B.

FIGS. 11A-11C are enlarged versions of portions of FIGS. 10A-10Crespectively illustrating the areas surrounding the icon.

FIGS. 12A-12C show masks used in applying the overlay method of thepresent invention to the transitions shown in FIGS. 11A-11Crespectively.

FIG. 13 is a graph of white state reflectance against time for a whitepixels and shows typical white state drift in a background pixel, aproblem which may be reduced or eliminated by the drift compensationmethod of the present invention.

FIG. 14 shows an image on a display affected by ghosting effects causedby white state drift such as that shown in FIG. 13.

FIG. 15 shows a waveform suitable for use in the drift compensationmethod of the present invention.

FIGS. 16A and 16B are pixels maps showing areas of background pixels towhich one step of the drift compensation method is to be applied, withFIG. 16A showing application of the step to 12.5 per cent, and FIG. 16Bshowing app of the step to 100 per cent, of the background pixels in theillustrated area.

FIG. 17 is a flow diagram showing the implementation of a preferreddrift compensation method of the present invention.

FIG. 18 is a graph similar to that of FIG. 13 but showing, in additionto the curve for an uncorrected pixel, curves for two different methodsof drift compensation in accordance with the present invention.

FIGS. 19A and 19B are images similar to that of FIG. 14, with FIG. 19Abeing an uncorrected image and FIG. 19B being an image corrected by adrift compensation method of the invention.

FIG. 20 is a graph similar to that of FIG. 18 and again showing curvesfor both uncorrected and corrected pixels.

FIG. 21 is a graph of remnant voltage against time (expressed as numberof cycles) for both uncorrected pixels and pixels corrected using adrift compensation method of the invention.

DETAILED DESCRIPTION

As will be apparent from the foregoing, the present invention provides anumber of improvements in the driving of electro-optic displays,especially bistable electro-optic displays, and most especiallyelectrophoretic displays, and in displays and components thereofarranged to carry out the improved method. The various improvementsprovided by the present invention will primarily be described separatelybelow but it should be noted that a single physical display or componentthereof may implement more than one of the improvements provided by thepresent invention. For example, it will readily be apparent to thoseskilled in the technology of electro-optic displays that the driftcompensation method of the present invention may be implemented in thesame physical display as any of the other methods of the presentinvention.

Part A: Update Buffer Invention

As already mentioned, the update buffer aspect of the present inventionprovides display controllers and methods for operating a display withthe T and F transitions already discussed. In one aspect, this aspectprovides a display controller having an update buffer, means forremoving from the update buffer pixels which do not require updatingduring a given transition, means for receiving a list of states thatshould not be removed from the update buffer, and means to ensure thatpixels having listed states are not removed from the update buffer. Forexample, consider the example given earlier of a controller in whichstates 1-16 correspond to the normal 16 gray levels, while state 17denotes a T transition and state 18 an F transition. In this case, thenumbers 17 and 18 are sent to the controller. If the controlleralgorithm recognizes a zero transition where the initial and finalstates are equal but on the list, the relevant pixel is not removed fromthe update buffer.

Another aspect of the update buffer invention provides a displaycontroller having an update buffer, and means for removing from theupdate buffer pixels which do not require updating during a giventransition, the controller having at least one special transition havingtwo states associated therewith, means to determine when a pixel isundergoing a special transition immediately after a previous specialtransition, and means to insert into the update buffer the second stateassociated with the at least one special transition when a pixel isundergoing a special transition immediately after a previous specialtransition. For example, consider a modification of the controllerdiscussed in the preceding paragraph in which states 1-16 correspond tothe normal 16 gray levels, while states 17 and 19 denote a T transitionand state 18 and 20 an F transition. The controller then operates suchthat if, at any specific pixel, the previous transition was aT-transition, and the next transition is also a T-transition, then thestate substituted into the image should be the second state associatedwith the T transition, namely 19. Thus, the pixel was assigned state 17for the previous transition but is assigned state 19 for the nexttransition. In this way the controller will always see specialtransitions as a change in state the associated pixels will never beflagged and removed from the update pipeline.

As already noted, the update buffer invention also provides a modifiedalgorithm for carrying out the SGU, BPPWWTDS or WWTOPDS drive schemesdiscussed above to take into account the non-flashing pixels that willbe introduced by the partial update mode of the controller. First, thePartial Update Mask (PUM) value for each pixel must be computedaccording to the known controller algorithm. In the simplest case(standard partial update) the PUM is set to False if and only if theinitial and final gray levels in the image buffer are the same. Second,a modified algorithm is used which utilizes the PUM to determine localactivity as prescribed by the algorithm. Pseudo-code for two suchalgorithms is provided below:

First algorithm Inputs: Initial (initial image pixels), Final (finalimage pixels), SFT (activity threshold), PUM (pixel update map) For allpixels in any order: If the pixel Initial to Final transition is notwhite-to-white, apply the standard GL transition. Else, If at least SFTcardinal neighbors (i.e., neighbors sharing a common edge) are not(making an Initial to Final transition from white-to-white OR have PUM =0), apply the F transition. Else, If all four cardinal neighbors have (aFinal gray level of white OR have PUM = 0), AND at least one cardinalneighbor has (an Initial gray level not white AND PUM = 1), apply the Ttransition. Otherwise use the empty (GL) W -> W transition. End

Second algorithm Inputs: Initial (initial image pixels), Final (finalimage pixels), AM (active mask) SFT (activity threshold), PUM (pixelupdate map) For all pixels in any order: If the pixel Initial to Finaltransition is not white-to-white, apply the standard GL transition.Else, If the pixel is selected by the AM, apply the F transition. Else,If at least SFT cardinal neighbors (i.e., neighbors sharing a commonedge) are not (making an Initial to Final transition from white-to-whiteOR have PUM = 0), apply the F transition. Else, If all four cardinalneighbors have (a Final gray level of white OR PUM = 0), AND (at leastone cardinal neighbor has (an Initial gray level not white AND PUM = 1)OR (at least one cardinal neighbor is selected by the AM), apply the Ttransition. Otherwise use the empty (GL) W -> W transition. End

It may be desirable to use the algorithm in conjunction with a regionaldisplay mode of the controller. A preferred regional update area for anoverlaid item is the area of the item plus one pixel all around itsperiphery; in this one-pixel border area, the special transition foredge ghosting reduction will be applied when the overlaid item isremoved. One controller solution involves the following sequence ofactions based on a new controller functionality: creating a full screenimage combining the initial image with the addition of theitem→performing full screen image processing using that image and theprevious initial image based on waveform algorithm→make the decision toperform a regional update using the processed new image for the area andlocation of the item plus one pixel all around.

From the foregoing, it will be seen that the update buffer controllersand methods of the present invention provide a pathway to use the edgeand areal ghosting artifact reducing waveform techniques described inthe aforementioned US 2013/0194250 on controllers that implement a“partial update” mode. The present invention requires only a smallmodification of the definition of the waveform states and a modificationof the algorithm, without any changes to controller functionality.

Part B: BPPTOPWWTDS Invention

As already mentioned, the BPPTOPWWTDS aspect of the present inventionprovides a balanced pulse pair/top-off pulse white/white transitiondrive scheme in which pixels undergoing white-to-white transitions,identified as likely to give rise to edge artifacts, and in aspatio-temporal configuration such that the drive scheme will beefficacious in erasing or reducing the edge artifact, are driven using awaveform which comprises at least one balanced pulse pair and at leastone top-off pulse.

A preferred white-to-white waveform for a BPPTOPWWTDS of the presentinvention is illustrated in FIG. 4 of the accompanying drawings. As maybe seen from FIG. 4, the waveform comprises an initial top-off pulse inthe form of a single negative (white-going) frame, followed by two frameof zero voltage, and four successive balanced pulse pairs, each of whichcomprises a positive (black-going) frame followed immediately by anegative (white-going) one.

The use of a BPPTOPWWTDS of the present invention has been shown to bevery effective in significantly reducing all edge artifacts, asillustrated in FIG. 5, which should be compared with the similarmicrograph shown in FIG. 2; it will be seen that essentially no edgeartifacts are present on the right side of FIG. 5, in contrast to thevery prominent edge artifacts visible on the right side of FIG. 2. As aresult, the performance of non-flashy drive schemes aimed at maintainingthe background white state lightness level can be significantly improvedwith observed decrease in white state lightness level of less than 0.5L* using a BPPTOPWWTDS of the present invention versus over 3L* using aprior art BPPWWTDS after 24 updates at 45° C. as shown in FIG. 6.

Preferred embodiments of a BPPTOPWWTDS of the present invention, usingonly a single top-off pulse, but varying the number of balanced pulsepairs and the location of the top-off pulse relative to the balancedpulse pairs, have been observed to provide a wide range of possiblewaveform solutions for operating at from 28° C. to 45° C., asillustrated in FIG. 7. In this case, acceptable solutions correspond tothose resulting in zero delta L* after 24 updates in special low flashmode using the BPPTOPWWTDS. The most significant tuning elements are thelocation of the top-off pulse relative to the BPP's and the number ofBPP's, with a small degree of tunability provided by the location of theBPP's. Locating the top-off pulse closer to the BPP's results in morepositive delta L* solutions, with the optimal location being the frameright after the BPPs. For a given BPPTOPWWTDS white-to-white waveform,it has been observed that decreasing the temperature results in morepositive delta L*. Although a potential problem could be that theBPPTOPWWTDS could create solutions with too positive a delta L* (meaningthat the display becomes whiter and whiter in an uncontrolled manner),it has been possible to avoid this problem by simply increasing thenumber of BPP's in the white-to-white waveform which results in a lesspositive delta L*. FIG. 7 shows that the BPPTOPWWTDS of the presentinvention can provide good results over the temperature range of 28° C.to 45° C., while still allowing enough tunability to account for modulevariability experienced in commercial mass production of electrophoreticdisplays.

The presence of the top-off pulse in the BPPTOPWWTDS of the presentinvention renders the drive scheme somewhat DC imbalanced, and (asdiscussed in several of the aforementioned MEDEOD applications), DCimbalanced drive schemes are known to potentially cause significantdisplay reliability issues and significant changes in drive schemeperformance. However, as already noted significant reduction in edgeartifacts in electrophoretic displays can be achieved using just onetop-off pulse in the BPPTOPWWTDS white-to-white waveform, resulting(typically) in a mild DC imbalance of just one white-going frame. Usagereliability experiments using a special low flash mode that make use ofsuch a BPPTOPWWTDS have been conducted, and the results are shown inFIG. 8. As shown in that Figure, after over 50,000 updates (estimated tocorrespond to about one year of e-reader usage), only slight shifts ingray levels of between +0.2L* and −1.2L* were visible, and these slightshifts could be due to other known factors such as so-called displayfatigue. These results after over 50,000 updates also show variations inwhite state and dark state 30 second transient drifts of less than0.5L*. These results show that BPPTOPWWTDS with one top-off pulse usedin special low flash modes aimed at reducing edge artifacts andmaintaining background white state do not cause reliability issues. Thisis due to the drive scheme being only slightly DC imbalanced and beingused on the display in such a way that the potential effects of DCimbalance are contained.

From the foregoing, it will be seen that the BPPTOPWWTDS of the presentinvention can significantly extend the temperature range over whichelectrophoretic displays can operate without producing image defects, beenabling such displays to operate for a large number of updates at thetemperature range of about 30 to 45° C. without being subject to thetype of image defects to which prior art displays are subject, thusrendering displays using the drive scheme more attractive to users.

Part C: Overlay Invention

As already mentioned, the overlay aspect of the present inventionprovides a method for overlaying an item (an icon, menu, etc.) overexisting text or image content followed by a removal of the item, anddiffers from standard partial updates drive schemes in that only thepixels in the region of the item perform transitions (including selftransitions) in order to avoid text thinning/fading for text thatoverlaps with the item and to avoid seeing the text outside that areaflashing on to itself. In a simple form of the present invention aregional update is performed in the area of the overlaid item. Preferredvariants of the overlay method can allow for transparent areas withinthe overlaid item.

In the preferred variant of the overlay method discussed above withreference to FIGS. 11A-11C and 12A-12C, the algorithm used can besummarized as follows:

-   -   For a given pixel with a given gray level in the current image:        -   IF Mask determines that this pixel must perform a non-empty            self transition to update to the next image, set pixel state            on the next image to gray level state with non-empty self            transition;        -   ELSE set pixel state on next image to gray level state with            empty self transition.

In driving modes that involve 16 gray levels plus special statesrequired for special algorithms (for example, the “balanced pulse pairwhite/white transition drive scheme” and the “white/white top-off pulsedrive scheme” described in the aforementioned US 2013/0194250), thefive-bit drive scheme solution described above cannot be applied to all16 gray levels because a five-bit drive scheme does not provide enoughadditional states. The five-bit drive scheme solution may be appliedonly to a restricted number of gray levels. For example, if thealgorithm requires two special states then two gray levels must bedropped from the algorithm, so that, for example, the algorithm might beapplied to gray levels 1→14 only. Such a variant of the overlay methodcould still be effective is reducing text thinning/fading since most ofthe gray levels in text are with gray levels 1→14. However, in someother scenarios, restricting the process to certain gray levels mightnot work well enough.

In such cases, where selective partial updates are needed for allexisting gray levels, a “controller-spoofing” method can be used inconjunction with the algorithm described above. In such acontroller-spoofing method, all the pixels requiring empty selftransitions as determined by a mask are set to one same special emptystate with empty self transition (for example state 2). That processedimage is then sent to the controller using a special mode that has afully empty waveform in order to set the states inside the controller asdesired by the algorithm without actually updating the pixels of thedisplay. The second image is then displayed with the use of the specialempty state 2. Once it is desired to not do empty self transitions forpixels currently in state 2, or to do other transitions to other graylevels, another processed image needs to be sent to the controller withan empty waveform in order to reset all the pixels currently in state 2to their original states. Therefore, this solution could result inlatency issues as it requires sending to the controller two additionalprocessed images with empty waveforms.

In another variant of the overlay method, a device controller functionis provided which accepts the mask described above and places pixels onthe update buffer according to this mask instead of the partial updatelogic that it would normally perform. One shortcoming of this approachis the need for a mask of the opaque part of the overlaid item. This,however, is not an unrealistic requirement since the rendering enginefor the graphic user interface of a electro-optic display must have sucha mask available to it, but the use of such a mask does require agreater amount of data handling and increases system complexity.

An alternative to this mask-based approach is to determine the list ofpixels with self-transitions that should be refreshed based on theactivity of neighboring pixels, i.e., the mask is inferred from theimage data, and subsequent steps implement the approach as if it weremask-based. For example, one algorithm may be defined as:

-   -   For a given pixel with a given gray level in the current image:        -   IF it is determined from the next image that this pixel is            performing a self-transition to update to the next image AND            IF at least one of its cardinal neighbors (i.e., neighbors            sharing a common edge) is not performing a self-transition;        -   THEN set pixel state on the next image to gray level state            with non-empty self-transition        -   ELSE set pixel state on next image to gray level state with            empty self-transition.

Such an algorithm should be applied in a non-recursive manner in orderto avoid a propagation effect, i.e., setting a pixel to perform anon-empty self transition as determined from this algorithm would nottrigger setting its cardinal neighbors with self transitions to performnon-empty self transitions. For example, if a feature contains severalcolumns of pixels that are performing self transitions in an imagesequence while an icon is being overlaid and dismissed multiple times ontop of that feature, this algorithm would trigger the columns of pixelsat the edge of the feature to perform non-empty self transitions. Suchan approach should result in reducing most of the visible textthinning/fading as blooming typically affects only the immediatecardinal neighbors.

The algorithm described above is general in the sense that it is appliedto all gray levels, including white, and thus in a partial update modein which the background white state is not intended to flash, some whitepixels in the background may perform white→white transitions dependingon the activity of their neighboring pixels. For example, if a longblack line is written on the display, all the neighboring pixels aroundthe black line would perform white→white transitions, resulting in linesand geometric features with uniform thicknesses, thus avoiding the issueof non-uniform line thickness which has plagued prior art partial updatedrive schemes. However, the pixels performing white→white transitionsmay induce the formation of edge artifacts around them. Therefore,desirably such a drive method would be applied in conjunction with adisplay mode designed to reduce edge artifacts in order to avoid theformation of those artifacts. Another variation of this method wouldexcept certain gray levels; for example, the method could be applied toall gray levels except white, thus avoiding the aforementioned edgeartifact problem.

In the method just above, as in the mask-based method previousdescribed, a five-bit drive scheme may be used if only 16 gray levelsare required. If additional special states exist in the drive scheme,the method may be applied to most but not all of the gray levels, forexample gray levels 1→14 of 16. As with the mask-based approach, thisdrive scheme would solve most of the text thinning/fading issues. If itis necessary to apply the method to all existing states, then theimplementation of this method would require resetting the states insidethe controller as described previously with the use of two additionalempty display updates.

From the foregoing, it will be seen that the method of the presentinvention can reduce or eliminate problems such as text thinning andfading encountered in prior art partial update drive schemes, whilemaintaining the low-flash characteristics of a partial update drivescheme for electro-optic displays. The present method is compatible withnovel drive scheme algorithms that result in low-flash, high imagequality display performance, thus rendering displays using the drivescheme very attractive to users.

Part D: Drift Compensation Invention

As already mentioned, the drift compensation aspect of the inventionprovides a method of driving a bistable electro-optic display having aplurality of pixels each capable of displaying two extreme opticalstates, in which, after the display has been left undriven for a periodof time, successive refresh pulses are applied to proportions of thebackground pixels to reverse at least partially the effects of drift.

The drift compensation method may be regarded as a combination of aspecially designed waveform with an algorithm and (desirably) a timer toactively compensate for the background white state (or other) drift asseen in some electro-optic and especially electrophoretic displays. Thespecial waveform is applied to selected pixels in the background whitestate when a triggering event occurs that is typically based on a timerin order to drive the white state reflectance up slightly in acontrolled manner.

One example of a waveform useful in the drift compensation method isshown in FIG. 15. This waveform may be as short as 2 frames (about 24milliseconds with a typical 85Hz frame rate) and may contain a singlewhite-going top-off pulse (frame 1). The purpose of this waveform is toslightly increase the background white state in a way that isessentially invisible to the user and therefore non-intrusive. The drivevoltage of the top-off pulse may be modulated (for example −10V insteadof the −15V used in other transitions) in order to control the amount ofwhite state increase.

In the drift compensation method of this invention the waveform of FIG.15 or a similar waveform is applied to selected pixels in the backgroundwhite state, thus allowing a control white state increase from theupdate, as illustrated in FIGS. 16A and 16B. By making use of a designedpixel map matrix (PMM) combined with an algorithm, the percentage of thepixels receiving a top-off pulse at each update is controlled. Thealgorithm used may be a simplified version of the algorithm described inthe aforementioned US 2013/0194250. The special transition shown in FIG.15 would correspond to the F W→W transition discussed in this publishedapplication.

Drift compensation is applied by requesting a special update to theimage currently displayed on the display. The special update calls aseparate mode storing a waveform that is empty for all transitions,except for the special transition shown in FIG. 15. The waveformalgorithm will select the pixels that will receive the top-off pulseusing the waveform algorithm described below. PMM_VS, PMM_HS, PMM_Periodare the vertical size, horizontal size, and period of the pixel mapmatrix. An update counter ensures that all the pixels will uniformlyreceive the same amount of top-off pulses over time. A typical algorithmis as follows:

Waveform algorithm for Active Drift Compensation with Timer Inputs:Current (current image pixels), Next (next image pixels equal to currentimage pixels), PMM (pixel map matrix) Set Active Mask (i, j) = TRUE ifPMM(i mod PMM_VS, j mod PMM_HS) == Update Counter mod PMM_Period For allpixels (i, j) in any order: If the pixel graytone transition is not W ->W, apply the standard transition. Else, if the pixel is selected byActive Mask (i, j), apply the F W -> W transition. Otherwise use thestandard transition. End

The drift compensation method very desirable incorporates the use of atimer. The special waveform used results in an increase in thebackground white state lightness. Therefore, if this update was tied touser-requested updates, there would be large variations in white stateincrease depending on how quickly updates were being requested, i.e., ifthis special update were applied every time a user requested an update,the white state increase would become unacceptably high if a user turnedpages very quickly (such as every one second), as opposed to a userturning pages more slowly (such as every thirty seconds). This wouldresult in the drift compensation method being very sensitive to dwelltimes between updates and in some cases unacceptably high ghosting wouldoccur due to the background white state being increased too much. Theuse of a timer decouples drift compensation from user-requested updates.By applying the special update independently of user-requested updates,the drift compensation is more controlled and less sensitive to dwelltimes.

A timer may be used in the drift compensation method in several ways. Atimeout value or timer period may function as an algorithm parameter;each time the timer reaches the timeout value or a multiple of the timerperiod, it triggers an event that requests the special update describedabove and resets the timer in the case of the timeout value. The timermay be reset when a full screen refresh (a global complete update) isrequested. The timeout value or timer period may vary with temperaturein order to accommodate the variation of drift with temperature. Analgorithm flag may be provided to prevent drift compensation beingapplied at temperatures at which it is not necessary. FIG. 17 is a flowdiagram of a drift compensation method implementing the conceptsdiscussed in this paragraph.

Another way of implementing drift compensation is to fix the timerperiod TIMER_PERIOD (for example, at 60 seconds), and make use of thealgorithm PMM and PMM_PERIOD to provide more flexibility as to when thespecial update is applied. For example, for PMM=[1], PMM_PERIOD=4 andTIMER_PERIOD=60, this is equivalent to applying a top-off pulse to allthe background pixels every 4×60 seconds. Other variations may includeusing the timer information in conjunction with the time since the lastuser-requested page turn. For example, if the user has not requestedpage turns for some time, application of top-off pulses may cease aftera predetermined maximum time. Alternatively, the top-off pulse could becombined with a user-requested update. By using a timer to keep track ofthe elapsed time since the last page turn and the elapsed time since thelast application of a top-off pulse, one could determine whether toapply a top-off pulse in this update or not. This would remove theconstraint of applying this special update in the background, and may bepreferable or easier to implement in some cases.

Examples of the background white state over time with and without driftcompensation are shown in FIG. 18. The lowest curve (similar to thatshown in FIG. 13) shows the uncorrected background white state over thecourse of 45 page turns at 30 second intervals. The illustrated drop inwhite state reflectance would result in substantial text ghosting overtime. The center curve shows the result of drift compensation in which12.5% of the pixels receive the special update every minute, again while45 page turns occur at 30 second intervals. The upper curve shows asecond example of drift compensation in which 100% of the pixels receivethe special update every six minutes, using the same sequence of pageturns. In both drift compensated cases, the background white state ismaintained at a higher level over time which will result in reduced textghosting and may allow to achieving a higher number of page turnswithout a full display refresh. In both cases, the special updates havebeen shown to be invisible to the user. The timer period may be used asanother way to control how much white state increase is being appliedoverall. The improvement in text ghosting is illustrated in FIGS. 19Aand 19B, with FIG. 19A showing the uncorrected display at the end of thesequence of page turns and FIG. 19B the display in which 100% of thepixels receive the special update every six minutes.

As indicated previously, the white state drift correction may be tunedby a combination of the pixel map matrix, the timer period, and thedrive voltage for the top-off pulse. FIG. 20 illustrates the tuning ofthe background white state drift by varying the density of the pixel mapmatrix from 12.5% to 50% with a fixed timer of three minutes, using thesame sequence of page turns as in FIG. 18.

As already mentioned, the use of DC imbalanced waveforms is known tohave the potential to cause problems in bistable displays; such problemsmay include shifts in optical states over time that will cause increasedghosting, and in extreme cases may cause the display to show severeoptical kickback and even to stop functioning. This is believed to berelated to the build up of a remnant voltage or residual charge acrossthe electro-optic layer, and this remnant voltage has a very long decaytime. Therefore, it is important to consider the effect of driftcompensation on remnant voltage. FIG. 21 shows curves of remnant voltageagainst time for an uncorrected pixel and three pixels using differentdrift compensation methods for the same sequence of page turns as inFIG. 20. FIG. 21 shows that in the worst case, drift compensationresults in an increase of remnant voltage of about 100 mV above thebaseline. Prior knowledge indicates that remnant voltages within awindow of about ±250 mV are typical in normal usage. Therefore, FIG. 21indicates that drift compensation does not appear to have a significantimpact on the remnant voltage, and therefore on display reliability withusage.

As already indicated drift compensation can be applied to dark statedrift as well as white state drift. A typical waveform for dark statedrift compensation could be simply the inverse of that shown in FIG. 15,with a single frame of positive voltage.

From the foregoing it will be seen that the drift compensation method ofthe present invention provides a means for substantially reducing theeffects of drift on a displayed image in a manner which is typicallyunnoticeable to a user and which does not adversely affect the long termuse of the display.

The methods of the present invention may be “tuned” to produce accurategray levels using any of the techniques described in the aforementionedMEDEOD applications. Thus, for example, the waveform used may includedrive pulses having a polarity opposite to that of the waveform as awhole. For example, when a pixel is driven from white to a light graylevel, the waveform will typically have an overall black-going polarity.However, to ensure accurate control of the final light gray level, itmay be desirable to include at least one white-going pulse in thewaveform. Furthermore, for similar reasons, as discussed in theaforementioned MEDEOD applications, it is often desirable to include atleast one balanced pulse pair (a pair of drive pulses of substantiallyequal absolute impulse value, but of opposite polarity) and/or at leastone period of zero voltage in the waveform.

It will be apparent to those skilled in the art that numerous changesand modifications can be made in the specific embodiments of theinvention described above without departing from the scope of theinvention. Accordingly, the whole of the foregoing description is to beinterpreted in an illustrative and not in a limitative sense.

1. A display controller capable of controlling the operation of abistable electro-optic display and having an update buffer, means forremoving from the update buffer pixels which do not require updatingduring a given transition, means for receiving a list of states thatshould not be removed from the update buffer, and means to ensure thatpixels having listed states are not removed from the update buffer.
 2. Adisplay controller capable of controlling the operation of a bistableelectro-optic display and having an update buffer, and means forremoving from the update buffer pixels which do not require updatingduring a given transition, the controller having at least one specialtransition having two states associated therewith, means to determinewhen a pixel is undergoing a special transition immediately after aprevious special transition, and means to insert into the update bufferthe second state associated with the at least one special transitionwhen a pixel is undergoing a special transition immediately after aprevious special transition.
 3. A display controller according to claim1 wherein the specified drive method is applied only to pixels withinone or more defined regions of the display and a different drive schemeis applied to pixels outside the defined region or regions.
 4. Abistable electro-optic display comprising a drive controller accordingto claim
 1. 5. An electro-optic display according to claim 4 comprisinga rotating bichromal member, electrochromic or electro-wetting material.6. An electro-optic display according to claim 4 comprising anelectrophoretic material comprising a plurality of electrically chargedparticles disposed in a fluid and capable of moving through the fluidunder the influence of an electric field.
 7. An electro-optic displayaccording to claim 6 wherein the electrically charged particles and thefluid are confined within a plurality of capsules or microcells.
 8. Anelectro-optic display according to claim 6 wherein the electricallycharged particles and the fluid are present as a plurality of discretedroplets surrounded by a continuous phase comprising a polymericmaterial.
 9. An electro-optic display according to claim 6 wherein thefluid is gaseous.
 10. An electronic book reader, portable computer,tablet computer, cellular telephone, smart card, sign, watch, shelflabel, variable transmission window or flash drive comprising a displayaccording to claim
 4. 11. A method of driving a bistable electro-opticdisplay having a plurality of pixels wherein, in a pixel undergoing awhite-to-white transition and lying adjacent at least one other pixelundergoing a readily visible transition, there is applied to the pixelone or more balanced pulse pairs and at least one top-off pulse, whereineach balanced pulse pair comprises a pair of drive pulses of opposingpolarities such that the net impulse of the balanced pulse pair issubstantially zero.
 12. A method according to claim 11 wherein thecombination of one or more balanced pulse pairs and at least one top-offpulse is applied only when the display is above a predeterminedtemperature.
 13. A method according to claim 11 wherein the top-offpulse comprises a single white-going drive pulse.
 14. A method accordingto claim 11 where only a single top-off pulse is applied to the pixel.15. A display controller or electro-optic display arranged to carry outthe method of claim
 11. 16. A method of driving a bistable electro-opticdisplay comprising overlaying a non-rectangular item over pre-existingtext or image content followed by removal of the item, wherein onlypixels in the region of the item perform transitions (includingself-transitions).
 17. A method according to claim 16 wherein the itemincludes at least one transparent region, and wherein only pixels in thenon-transparent regions of the item perform transitions (includingself-transitions).
 18. A display controller or electro-optic displayarranged to carry out the method of claim 16 or 17.